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What is the best way to reduce CO 2 emissions in housing, and what levels of insulation should architects be encouraging clients to specify? Over and above the requirements of the new Part L?

Do increased levels of insulation invariably lead to reduced CO 2 emissions? And when does the law of diminishing returns kick in?

Until recently, such questions tended to be answered by means of the steady-state analysis of U-values, but the introduction of dynamic computer modelling enables the true thermal performance of buildings to be assessed more accurately.

Not surprisingly, the results are more complex than the steadystate model indicates, and in some cases are counter-intuitive.

Research by the University of Glamorgan using TAS software from Environmental Design Solutions took a series of insulation thicknesses for floors, walls and roofs of a typical fourbedroom detached house. The model was run over the heating season for each thickness of insulation, using CIBSE climatic data for Cardiff and including in the model the effect of heat gains, including solar gain and those from lighting, cooking, showers and occupants. Conservative assumptions were made about the boiler (taken as a 25kW SEDBUK Category B 86 per cent efficient gas-condensing model), the infiltration rate (one air change an hour) and the length of the heating season.

Three different levels of insulation were chosen: the Part L1 2002 standard; the Energy Saving Trust's (EST) 'best practice' standard for new dwellings, based on the new Part L; and the EST's 'advanced standard'.

These were then incorporated into standard cavity-wall, solid-floor and cold-pitched roof constructions, the only other variables between the simulations being that the two EST standards required slightly thicker blockwork to the inner leaf of the cavity walls (in order to keep cavities to a minimum).

The results for CO 2 emissions and associated fuel costs over a year are shown in the charts opposite. Analysis showed that:

In the case of cavity-wall insulation, the reductions in emissions and fuel costs are less significant with increases in insulation thicker than 50mm. Other analysis by the University of Glamorgan has shown that, for instance, increasing the thickness of urethane insulation from 40mm to 60mm produces only a 6 per cent decrease in CO 2 emissions, and adding another 20mm beyond that effects only a 1 per cent further reduction. In other words, the law of diminishing returns kicks in forcefully at around 50mm of urethane insulation increasing thicknesses much beyond that is not effective;

With loft insulation, no significant reductions occur beyond an insulation thickness of 250mm, ie 2002 Part L standards; and - In the case of solid-floor insulation, adding insulation actually increases emissions and fuel costs.

Some of these results fit well with historic research. In 1984, the optimum level of mineral wool for lofts in a typical dwelling was identified by the Building Research Establishment (Report No 58). Comparisons were made between two common house constructions for varying fuel types, interest rates, and payback periods, with the conclusions ranging from 125mm to 200mm. European research regarding cavity-wall insulation has placed the optimum thickness at between 75mm and 170mm of mineral wool. U-values achievable with these thicknesses are largely reflected in the 2002 Part L requirements.

Solid floors are not so easily dealt with, and the calculation of heat loss (and therefore the optimisation of insulation thickness) has been the subject of much controversy.

It is generally agreed that the perimeter of a floor requires some insulation but, conversely, in 1990 it was asserted in a Building Services Journal article that complications may arise if insulation thicknesses become too great, arguing that heat loss may actually increase as the ability of the ground to act as a heat store starts to be compromised.

How do the results impact upon wider issues of energy efficiency? Some initial investigations at Kingston University into property values and floor areas have looked at the loss of floor space or increase in costs that occur if wall insulation thicknesses are increased substantially. For an inner-city, three-storey development on a tight site, with three one-bedroom flats of around 45m 2 each, increasing the wall thickness from 300mm to 400mm reduces the internal floor area of the flats to 42m 2. This represents a loss of 3m 2 over each of the three floors.

Purchase costs in, say, south London are around £3,600 per m 2, resulting in an increased cost of £10,000 to a purchaser, or a concomitant reduction in the size of the flat, not taking into account the additional construction costs associated with increased wall thicknesses. Given that room sizes and affordability are important indices of the overall sustainability of a development, and that thermal modelling shows negligible benefits for increasing wall thicknesses to this extent, it appears doubtful whether housing design should, in general, follow the exhortations of those who argue for, say, 300mm of wall insulation.

The practical effect of these results in respect of new housing is to encourage architects and environmental engineers to focus - once the insulation levels required by Part L are satisfied - on other methods of reducing emissions and fuel costs when encouraging their clients towards excellent practice.

Investment in boiler efficiency, passive solar techniques and air infiltration are likely to be relatively more effective. In respect of the upgrading of existing housing, the effectiveness of loft insulation is confirmed, as is the effectiveness of lining existing external walls (either internally or externally) with relatively thin layers (30mm to 50mm) of insulation.

They also place the standards required in the new Part L - in some quarters criticised as a missed opportunity for reducing emissions - in a new light.

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